This invention relates to glass-ceramics having a super flat polished surface and a ultra low thermal expansion property capable of being used as a substrate for a mirror used for super precision purposes such as a next generation semiconductor manufacturing equipment and also relates to a method for manufacturing such glass-ceramics.
In recent years, efforts have been actively made for development of a next generation semiconductor exposure techniques intended for realizing higher density integration of circuits on IC chips.
For realizing a higher density integration of circuits on IC chips, efforts are generally made for achieving finer exposure resolution which is expressed as R: exposure resolution represented by the formula R=k×(λ/NA) (where k is a constant, λ is exposure wavelength and NA is numerical aperture). It is a recent tendency to make an attempt for realizing finer exposure resolution by shortening the exposure wavelength.
There has actually been shifting in the exposure wavelength from so-called i-line (467 nm) and g-line (365 nm) to excimer lasers such as a KrF laser (248 nm) and an ArF laser (193 nm). Further, it has recently been made possible to realize a fine width of exposure line in the order of 45 nm (so-called “45 nm node”) as the minimum width of exposure line by adopting liquid immersion exposure technique which shortens apparent exposure wavelength by filling pure water in space between an objective lens and a silicon wafer which is an object to be exposed, or by combining with super resolution techniques such as OPC (optical proximity correction) or PSM (phase shift mask).
However, for coping with realizing a finer width of exposure line with the liquid immersion technique in a coming 32 nm node generation which will follow the 45 nm node generation, a liquid which has a larger refractive index than pure water and which will not contaminate other exposure system must be developed. Moreover, the super resolution techniques will become more complex than in the present stage, which will incur increase in the cost for exposure.
Extreme ultraviolet ray exposure technique (EUVL) using extreme ultraviolet ray as a light source uses light having wavelength of only 13 nm which is very short compared with light used in the conventional optical lithography and the exposure resolution represented by R can thereby be made very small. For this reason, EUVL is thought to be a hopeful exposure technique in the coming 32 nm mode and subsequent generations.
Since the wavelength used in EUVL is a very short one of 13 nm and has a large absorption coefficient to lens materials used in the current optical lithography, such optical system needs to employ not a transmitted light system which has been used in the conventional optical lithography but a reflected light system using a mirror.
In the reflected light system which can be used in EUVL, if a projected image on the mirror surface is distorted due, for example, to thermal expansion of a substrate material, it will ultimately result in deterioration in the exposure quality. It is therefore necessary to use a material having an extremely small thermal expansion coefficient as substrates of a mirror and a photo mask used in EUVL.
Further, super flatness of an atom level, i.e., Rms of a component having coarseness expressed by spatial frequency of 10 μm or below is 0.15 nm or below sa described in literature (SEMI P37-1102), is sought for a surface of substrates of such mirror and photo mask used in the reflected light system of EUVL.
For coping with this problem, it is conceivable to use titan-dope silica glass produced by vapor phase synthesis or ultra low thermal expansion glass-ceramics as a substrate material. However, titan-dope silica glass which is currently commercially available has inhomogeneity in its composition caused during the vapor phase synthesis and, as a result, the thermal expansion coefficient on the surface of the material becomes inhomogeneous and therefore is unsuitable for a substrate of a mirror used in EUVL which is required to have severe CTE homogeneity in the order of a ppb/K level.
Glass-ceramics having β-quartz crystal as a predominant crystal phase which are produced by heat treating a Li2O—Al2O3—SiO2 system glass exhibits an ultra low thermal expansion property and a fine structure of a submicron order and, advantageously, is mostly produced by a melting process. For these reasons, the glass-ceramics are superior in optical homogeneity to the above described titan-dope silica glass.
In a case where such ultra low thermal expansion glass-ceramics are used as a mirror substrate, their surface roughness after polishing generally depends upon crystal grain diameter of the glass-ceramics and, therefore, the surface roughness after polishing is expected to be made smaller by making the average crystal grain diameter. Since, however, polished surfaces of the EUVL mirror and photo mask substrate require super flatness as mentioned above, it will not suffice to consider only the average crystal grain diameter of the glass-ceramics.
Since the average crystal grain diameter is an average value of crystal grain diameters of glass-ceramics, even if a crystal grain having an extraordinarily large diameter exists in the glass-ceramics, this extraordinarily large crystal grain diameter is averaged with crystal grain diameters of other crystal grains in the glass-ceramics and, therefore, its existence will not significantly affect the average crystal grain diameter of the glass-ceramics. On the other hand, so far as surface roughness after polishing of the glass-ceramics is concerned, a peak to valley value of projections and depressions on the surface of the glass-ceramics will become large by existence of the crystal grain having such extraordinarily large crystal grain diameter and, as a result, the glass-ceramics will not be suitable for use as substrates for super precision purposes such as the EUVL mirror.
Accordingly, in developing glass-ceramics to be used as a substrate material of the EUVL mirror or photo mask substrate, not only an average crystal grain diameter but also a state of distribution of crystal grain diameters need to be controlled.
Japanese Patent Application Laid-open Publication No. 2005-089272 discloses ultra low thermal expansion glass-ceramics having ultra low thermal expansion property and transparency obtained by heat treating a Li2O—Al2O3—SiO2 glass containing TiO2 and ZrO2 as crystal precipitating agents. The glass-ceramics have surface roughness Ra of a polished surface of 0.3 nm or below.
This publication describes about an average crystal grain diameter within a range of 50 nm-90 nm as a factor for achieving flatness of the glass-ceramics but fails to discuss distribution of crystal grain diameters.
The inventor of the present invention has measured the distribution of crystal grain diameter of the examples of this publication and found that the examples all have substantially large distribution of crystal grain diameters and, therefore, there still is room for improvement in the glass-ceramics of the publication when utility of the glass-ceramics as a substrate material for the EUVL mirror or photo mask substrate is considered.
Japanese Patent Application Laid-open Publication No. 2005-063949 discloses ultra low thermal expansion glass-ceramics obtained by heat treating a Li2O—Al2O3—SiO2 glass containing TiO2 and ZrO2 as crystal precipitating agents. It is described that the glass-ceramics have a variation coefficient (distribution of crystal grain diameter/average crystal grain diameter shown by percentage) of 10% or below. Since, however, the average crystal grain diameter of the glass-ceramics spans a very wide range of 100 nm to 1000 nm and, moreover, no discussion is made about the surface roughness after polishing, utility of the glass-ceramics as a substrate material of the EUVL mirror or photo mask substrate is not clear.
Japanese Patent Application Laid-open Publication No. 2003-267789 discloses glass-ceramics having ultra low thermal expansion property and a variable CTE-T curve obtained by heat treating a Li2O—Al2O3—SiO2 glass containing TiO2 and ZrO2 as crystal precipitating agents. It is described that the value of CTE is variable as a result of adjustments in heat treatment temperature and time taken for crystallization in the crystallization process. Since, however, no discussion is made in the publication about the size of crystal grain diameter and distribution of crystal grain diameters and also surface roughness after polishing of the glass-ceramics, utility of the glass-ceramics as a substrate material of the EUVL mirror or photo mask substrate is not clear.
It is an object of the present invention to provide glass-ceramics having super flat surface roughness and ultra low thermal expansion property which have not been realized to date.
It is another object of the invention to provide a method for manufacturing such glass-ceramics.